New nonlinear organic materials are conveniently studied and characterized as thin film samples. The macroscopic nonlinear response described by the susceptibility tensor is closely related to the quality of the sample, ordering of molecules, and other properties of the film. In order to characterize the nonlinearity properly and to access the resulting information, the susceptibility tensor should be accurately determined. Unfortunately, this requires a theoretical model of the nonlinear interaction, where certain assumptions, whose validity is often difficult to verify, must be made. This may compromise the reliability of the results. We use a technique based on the polarization properties of second-harmonic generation to characterize thin films, which facilitates the verification of the assumptions while allowing the determination of the susceptibility tensor. Chiral molecules have no center of symmetry and are therefore naturally very interesting subjects of study in secondorder nonlinear optics. We apply our technique to chiral, anisotropic Langmuir-Blodgett films of helicene molecules to obtain the relative values of the components of their susceptibility tensors and estimates of their accuracy. Usually such samples are assumed to have C<sub>2</sub> symmetry, but we prepare samples having different structures and report results where the symmetry groups C<sub>2</sub> and D<sub>2</sub> can be distinguished. In addition, the orientation of the in-plane symmetry axis that appears in D<sub>2</sub> but is absent in C<sub>2</sub> is determined.